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| United States Patent |
5,653,379
|
|
Forster
, et al.
|
August 5, 1997
|
Clad metal substrate
Abstract
Ceramic to metal stock or substrates having a relatively large and thick
metal core and a relatively thin ceramic layer or layers are bonded to the
metal core or a selected portion thereof by providing a metal core
material having a temperature coefficient of expansion which is tailored
to the temperature coefficient of expansion of the ceramic layer to be
bonded thereto. The typical core materials include multilayer composite
metal laminates embodying Cu/Mo/Cu, Cu/Kovar/Cu, Cu/Invar/Cu and the like
and including powdered metal composites embodying Cu-W, Ag-Mo, Ag-W,
Al-Si, Cu/Mo/Cu, Cu/Kovar/Cu, SiC-Cu, Ni-Fe alloys having from about 20%
Ni to about 80% Ni, etc. The ceramic layer is chosen primarily for the
properties of dielectric strength and isolation properties and typically
include such ceramics as alumina, beryllium oxide, aluminum nitride,
silicon carbide, etc. Where the core composite includes copper outer
surface materials or is plated to have copper outer surface materials, the
core and ceramic materials and particularly multiple sections of the
ceramic materials are provided in spaced relation on the metal cores of
relatively large area and are preferably bonded to the core using a copper
oxide eutectic formed on the core surface.
| Inventors:
|
Forster; James (Barrington, RI);
Hingorany; Premkumar (Foxboro, MA);
Breit; Henry F. (Attleboro, MA)
|
| Assignee:
|
Texas Instruments Incorporated (Dallas, TX)
|
| Appl. No.:
|
459440 |
| Filed:
|
June 2, 1995 |
| Current U.S. Class: |
228/124.1; 228/122.1; 228/124.5 |
| Intern'l Class: |
B23K 031/02; B23K 035/24 |
| Field of Search: |
228/124.5,122.1,208,124.1
|
References Cited
U.S. Patent Documents
| 3744120 | Jul., 1973 | Burgess et al.
| |
| 3766634 | Oct., 1973 | Babcock et al.
| |
| 3854892 | Dec., 1974 | Burgess et al.
| |
| 3911553 | Oct., 1975 | Burgess et al.
| |
| 3993411 | Nov., 1976 | Babcock et al.
| |
| 3994430 | Nov., 1976 | Cusano et al.
| |
| 4129243 | Dec., 1978 | Cusano et al.
| |
| 4568586 | Feb., 1986 | Gobrecht.
| |
| 4611745 | Sep., 1986 | Nakahashi et al. | 228/124.
|
| 4763828 | Aug., 1988 | Fukaya et al. | 228/124.
|
| 4809135 | Feb., 1989 | Yerman.
| |
| 5082163 | Jan., 1992 | Kanahara et al. | 228/124.
|
| Foreign Patent Documents |
| 545179 | Aug., 1957 | CA | 228/124.
|
| 784931 | Oct., 1957 | GB | 228/124.
|
Primary Examiner: Bradley; P. Austin
Assistant Examiner: Knapp; Jeffrey T.
Attorney, Agent or Firm: Baumann; Russell E., Donaldson; Richard L., Grossman; Rene E.
Parent Case Text
This application is a division, of application Ser. No. 07/985,545, filed
Dec. 4, 1992 which is a continuation of Ser. No. 07/452,937 Dec. 18, 1989
now abandoned.
Claims
We claim:
1. A method of forming a substrate which includes the steps of:
(a) providing a first layer of ceramic material;
(b) providing a solid metal core having a coefficient of thermal expansion
substantially the same as that of said first layer and having at least one
copper surface, the ratio of the thickness of the metal core to that of
the layer of ceramic material being n:1 where n is a number equal to or
greater than about one;
(c) forming a copper-copper oxide eutectic material on said copper surface;
and
(d) joining said first layer to said copper surface of said solid metal
core by bonding said copper-copper oxide eutectic material to said first
layer and to said copper surface.
2. The method of claim 1 wherein said ceramic material is taken from the
class consisting of alumina, beryllium oxide, aluminum nitride and silicon
carbide.
3. The method of claim 1 wherein said metal core is taken from the class
consisting of Cu/Invar/Cu, Cu-W, Ag-Mo, Ag-W, Ag/Invar/Ag, Al-Si,
Cu/Mo/Cu, Cu/Kovar/Cu, SiC-Cu and Ni-Fe alloys having from about 20% Ni to
about 80% Ni, said metal core having a layer of copper surface material
thereon.
4. The method of claim 3 wherein said ceramic material is taken from the
class consisting of alumina, beryllium oxide, aluminum nitride and silicon
carbide.
5. The method of claim 1 further including providing a second layer of
ceramic material having a coefficient of thermal expansion substantially
the same as that of said metal core and joining said second layer to a
second copper surface of said metal core opposing said first surface by
bonding a copper-copper oxide eutectic material to said second layer and
to said second copper surface, the ratio of the thickness of the metal
core to that of said first and second layers of ceramic material being
1:n:1 where n represents the thickness of said core and is a number equal
to or greater than about one.
6. The method of claim 5 wherein said ceramic material is taken from the
class consisting of alumina, beryllium oxide, aluminum nitride and silicon
carbide.
7. The method of claim 5 wherein said metal core is taken from the class
consisting of Cu/Invar/Cu, Cu-W, Ag-Mo, Ag-W, Ag/Invar/Ag, Al-Si,
Cu/Mo/Cu, Cu/Kovar/Cu, SiC-Cu and Ni-Fe alloys having from about 20% Ni to
about 80% Ni, said metal core having a layer of copper surface material
thereon.
8. The method of claim 7 wherein said ceramic material is taken from the
class consisting of alumina, beryllium oxide, aluminum nitride and silicon
carbide.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a ceramic insulator coated metal and, more
specifically, to a large and thick clad metal member bonded to a
relatively thin electrically insulating ceramic material.
2. Brief Description of the Prior Art
Bonding of ceramic materials, such as alumina or the like to metal is
generally known in the prior art. Examples of procedures for direct
bonding of copper or other metals onto various ceramic materials is set
forth, for example, in U.S. Pat. Nos. 3,744,120, 3,766,634, 3,854,892,
3,911,553, 3,993,411, 3,994,430 and 4,129,243, and an article in IEEE
Transactions on Components, Hybrids and Manufacturing Technology entitled
"Thick Film and Direct Bond Copper Forming Technologies for Aluminum
Nitride Substrate" by Nobuo Iwase et al., June, 1985, the subject matter
of all of which is incorporated herein by reference.
Where such systems for bonding ceramic and metal materials as described in
the above noted patents have been proposed, they have suffered from the
limitation that mismatch in the temperature coefficient of expansion (TCE)
of ceramic (typically but not limited to alumina, beryllium oxide,
aluminum nitride, etc.) and metal (typically copper or high thermal
conductivity metals) does not permit relatively large or thick high
thermal conductivity metals to be bonded to relatively thin ceramic
insulating materials. As an example, the TCE of copper is 16 ppm/oC.
whereas the TCE for alumina is 7 ppm/oC., this mismatch resulting in the
application of great stresses to the materials with potential cracking of
the ceramic material. Accordingly, in accordance with the prior art as
exemplified by some of the above noted patents, in order to minimize this
problem, the amount of metal proposed for use in conjunction with the
ceramic in applications of the type described in the above noted patents
is very small. Typically, not more than 0.010 inch copper foil is bonded
onto a 0.025 inch thick or thicker alumina member and typically the area
or size of the composite has been limited to avoid cracking of the alumina
member. In such prior art systems, the core material is generally a
relatively thick ceramic material having a metal layer thereon rather than
a thick metal core material with the relatively thin layers of ceramic on
one or both sides of the metal material.
It is desirable to provide a relatively thick metal (as opposed to the
prior art thin metal) core having a relatively thin electrically
insulating layer or layers (as opposed to the prior art thick electrically
insulating layer) joined thereto in an economical and reliable way whereby
the metal core provides the highly beneficial properties of good thermal
conductivity by being a good heat sink for components disposed either on
the insulating layer or on the metal core itself. This arrangement
provides improved rigidity wherein a relatively thick breakable ceramic
layer is no longer present, particularly where the metal core has a
plurality of separate ceramic sections formed thereon and is capable of
providing larger size substrates and wherein the insulating layer will be
less subject to cracking due to relatively small thermal mismatch between
the metal core and the ceramic layer.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a method of
producing ceramic bonded to metal and preferable composite metal systems
wherein the ceramic layer is thin relative to the metal core thickness,
yet without cracking the ceramic layer or adversely affecting thermal
expansion of the composite metal and ceramic layers. Composite metals
include, but are not limited to, any combination of metals formed by
cladding, powder metallurgy or metal matrix composites such that the
composite metal material has controlled expansion and high thermal
conductivity.
Briefly, the metal core is defined as any clad, laminated and/or powder
metallurgically produced material of suitably high thermal conductivity
whose coefficient of thermal expansion is pre-controlled to substantially
match the coefficient of thermal expansion of the ceramic layer intended
to be bonded to the metal core. characteristics are determined by the
"Rule of Mixtures" and formula set forth by R. D. Delagi in "Designing
with Clad Metals", Machine Design, Nov. 20, 1980. The thermal expansion of
a laminate parallel to the surface is given by:
##EQU1##
where .varies..sub.1 and .varies..sub.2 are the coefficients of expansion
E.sub.1 and E.sub.2 are the moduli of elasticity and
f.sub.1 and f.sub.2 are the component thicknesses as a percentage of the
total thickness.
For example, a laminate of copper clad invar with a thickness ratio of
20/60/20 with:
.varies.copper=16.5.times.10.sup.-6
.varies.invar=1.6.times.10.sup.-6
Ecopper=17.times.106 psi
EInvar=20.5.times.106 psi
f.sub.1 copper=0.4
f.sub.2 Invar=0.6
would have an expansion of 6.3.times.10.sup.-6. Similar calculations can be
made for copper clad molybdenum, copper clad alloy 42, etc. In all cases
the ratio of the laminate can be tailored to provide a specific
coefficient of thermal expansion.
Although the discussion has been focused upon laminate systems, the
combination of materials could be produced using standard or conventional
powder metallurgical processes. These would include coblends of a variety
of powders which would then be pressed and sintered using techniques and
processes known to those skilled in the art. A number of theoretical
models are available to predict the performance of mixed powders, although
all models must be verified by experiments which could lead to a change of
mixtures to produce the desired properties. In any case, the combination
of two or more materials in specific ratios will produce a composite
material with an expansion which is between the values of the materials
being combined.
Examples of such metal cores are Cu/Invar/Cu, Cu-W, Ag-Mo, Ag-W,
Al/Invar/Al, Al/Invar, Al-Si, Cu/Mo/Cu, Cu/Kovar/Cu, Sic-Cu, Ni-Fe alloys
having from about 20% Ni to about 80% Ni, etc. The core can be a laminate
or a homogeneously mixed powder metallurgical system as shown in U.S. Pat.
No. 4,546,406 or in the commonly assigned copending application Ser. No.
07/166,290, filed Mar. 10, 1988, now U.S. Pat. No. 4,894,293. If a powder
metal material is used, the core surface is cleaned and coated by plating
or cladding with a thin copper layer. The ceramic material is chosen for
the properties of dielectric strength and isolation properties primarily.
Typical such ceramics are alumina, beryllium oxide, aluminum nitride,
silicon carbide, etc., with alumina being the presently preferred ceramic
material due to the large existing body of technology extant for
depositing electrical circuits onto alumina surfaces by thin film and
thick film techniques.
The metal core is bonded to the ceramic by soldering, brazing, direct
eutectic bond, adhesive bonding or any other technique where necessary
steps have been taken to produce good wettability on the ceramic surface,
some of the techniques which meet these requirements being set forth in
the above noted patents.
The technique used will depend upon the application and the structure of
the substrate material. For example, if the substrate is a composite of
aluminum and invar in such ratio that the expansion characteristics are
close to that of alumina, then pieces of alumina which have been prepared
with the necessary circuitry and electronic conductive paths, etc. can be
bonded using a thermally conductive epoxy, adhesive or solder. This
adhesive can be electrically conductive and may be filled with silver,
alumina or some other material for improved thermal characteristics.
In the case of eutectic formation, the eutectic is formed at a temperature
below the melting temperature of the metal core. Brazing and soldering
will also be performed below the melting point of the metal core. With the
ability to bond the thin ceramic layer onto the metal core, it is then
possible to utilize standard thin and/or thick film technology on the
insulating layer by screen printing or the like to form electric circuits,
etc.
The ratio of the thickness of the metal core to that of the insulator for
an insulator/core/insulator system in accordance with the present
invention would be 1:n:1 where n is a number equal to or greater than
about one (1). The maximum thickness of the core is essentially unlimited
and is a function of economics, space requirements and practicality.
Generally, substrates as contemplated herein would have a total thickness
in the vicinity of 0.010 inches though this dimension is not intended in
any way to be a limitation on the invention herein. In some applications
core thicknesses of 1/4 inch and even thicker are contemplated. The
preferred thickness ratio of insulator/core/insulator would be from about
1/3/1 to about 1/20/1. In the case of a non-symmetric combination of
insulator and core, the thickness of insulator to core would be 1:n where
n is a number equal to or greater than about one (1).
A substrate is produced in accordance with the present invention by
providing the desired core material, preferably Cu/Invar/Cu, with the
chemical formulation of the Invar having been adjusted so that the core as
a whole has a coefficient of thermal expansion closely matching that of
the contemplated ceramic material to be used, such procedures being known
in the art. A layer of that ceramic material is then formed over the core
material. This core-ceramic material is then processed in accordance with
any of the procedures and in conjunction with the materials herein wherein
as set forth, for example, in U.S. Pat. No. 3,993,411, a eutectic surface
layer, preferably in the form of cuprous oxide or the like, having the
properties of being stable, melting below the melting temperature of the
remainder of the core material and wetting the core material is formed.
The wetting agent must also have a melting point above the temperatures at
which later operations in conjunction with the substrate will take place,
such as, for example, thin or thick film operations. Upon the formation of
the cuprous oxide by reaction of the copper with the oxygen in a reactor
environment, a copper-cuprous oxide eutectic liquid is briefly formed to
wet the copper and the ceramic material and provide a good bond
therebetween.
A small amount of mismatch in the proper direction can be advantageous. If
mismatch is such that the ceramic material goes under slight compression
when cooled, then the result will be favorable. If the mismatch is such
that the ceramic goes into tension, the ceramic may then fail and crack.
This problem can be minimized by patterning the ceramic layer on the metal
core so that only selected area layers of ceramic are present on the core.
This arrangement will lessen the likelihood of the ceramic layer or layers
being placed under too severe an amount of tension and cracking. The
greater the mismatch, the smaller will be the size of the islands of
ceramic if the possibly objectionable results of any shift mismatch are to
be minimized.
Other advantages of controlled expansion metallic substrates are that they
can be used as carriers for ceramic substrates to improve ceramic
substrate thermal conductivity and rigidity and provide for disposition of
plural ceramic substrates thereon wherein the ceramic substrates with
electronics thereon are positioned on a single metal substrate with
possible electrical interconnection between substrates.
In addition, the rigid metal core can provide the necessary rigidity so
that the ceramic can be selectively machined to a thinner section in order
to improve the thermal properties of the system by such reduction of the
thickness of the thermal and electrical insulator. This machining can be
performed over the entire insulator surface to provide a very thin ceramic
coating or it can be performed selectively so that electronic devices can
be placed in areas where the insulator thickness is significantly reduced.
All of the selective machining is performed after all high temperature
operations have been completed so that differences in the TCE will not
lead to cracking of the ceramic.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a horizontal furnace which can be
used to provide a substrate in accordance with the present invention; and
FIGS. 2a and 2b are flow diagrams illustrating the processing steps
utilized to provide the substrate in accordance with two embodiments of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In accordance with the present invention, there is provided a core member
comprising a composite metal such as Cu/Invar/Cu, Cu-W, Ag-Mo, Ag-W,
Al/Invar/Al, Al/Invar, Al-Si, Cu/Mo/Cu, Cu/Kovar/Cu, SiC-Cu, Ni-Fe alloys
having from about 20% Ni to about 80% Ni, etc. The core can be a laminate
or a homogeneously mixed powder metallurgical system. A relatively thin
layer of ceramic material having a coefficient of thermal expansion
substantially the same as the core is then joined to the core to provide
the final substrate. Processing on the ceramic layer can take place before
or after joining to the core.
Referring first to FIG. 2a, there is shown a process flow diagram of the
processing steps required to produce substrates in accordance with the
present invention using copper eutectic direct bonding of ceramic to
controlled expansion substrates.
Initially, the controlled expansion material is fabricated in known manner,
such as by roll bonding, powder metallurgy and the like to prepare a
substrate of Cu/Invar/Cu. If the substrate is formed of powdered material,
the surface thereof is prepared by plating or roll bonding a thin layer of
copper thereon which is to be later oxidized. The copper surface is then
prepared using the techniques of the above noted General Electric Co.
patents to produce a copper oxide on the substrate surface. Alumina is
then laminated to the substrate and the lamina are joined together by
direct bonding techniques. The joined laminated substrates then prepared
for formation of electrical circuits thereon using standard thin film
techniques, thick film techniques or other appropriate electric circuit
forming techniques.
Referring now to FIG. 2b, there is shown a flow diagram showing the
processing steps required when using adhesive or soldering to bond the
ceramic substrate to the controlled expansion material.
Initially, the controlled expansion substrate is fabricated as in the
embodiment of FIG. 2a. The ceramic substrate is the formed and all
required thin and/or thick film processing thereon is provided. The mating
surfaces of the two substrates are then prepared for soldering and the
surface of the ceramic substrate is plated or enhanced to ensure
wettability. The substrates are then bonded together by an appropriate
joining technique.
The following is an example of a preferred embodiment showing formation of
substrates in accordance with the present invention:
EXAMPLE 1
Utilizing a system of the type set forth in FIG. 6 of U.S. Pat. No.
3,993,411 and FIG. 1 herein, there was provided a clad metal core 12 of
copper/invar/copper with layer thicknesses as in Example 1 and having a
TCE closely approximating that of the aluminum oxide ceramic material 13
to be bonded thereto positioned on the holder 26. The ceramic material 13
overlay the core 12. These materials were introduced into the quartz tube
22 through the opening 25 which was then sealed by suitable stopper means.
The quartz tube 22 was then purged with a reactive gas flow of
approximately 4 cubic feet per hour, for example. As used herein, reactive
gas flow or atmosphere means a mixture of an inert gas such as argon,
helium or nitrogen, for example, with a controlled minor amount of a
reactive gas, such as oxygen, a phosphorus-containing gas such as
phosphine, for example, or a sulfur-containing gas such as hydrogen
sulfide, for example. The amount or reactive gas in the total gas flow is
dependent, in part, on the materials to be bonded and the thickness of the
materials, in a manner more fully described hereinbelow. In general,
however, the partial pressure of the reactive gas must exceed the
equilibrium partial pressure of the reactive gas in the metal at or above
the eutectic temperature. For example, when bonding copper members to
refractory members in a reactive atmosphere including oxygen, the partial
pressure of oxygen must be above 1.5.times.10-6 atmosphere at the
copper-copper oxide eutectic temperature of 1065.degree. C.
After purging the quartz tube, the furnace was then brought to a
temperature sufficient to form the copper-copper oxide eutectic liquidus
or melt at the metal-substrate interface. For example, for a
copper-alumina bond with oxygen as the reactive gas, the temperature of
the furnace was brought to between approximately 1065.degree. and
1075.degree. C. Within this range of temperatures, a copper-copper oxide
eutectic formed on the copper member 13. This eutectic melt then wetted
the copper and the alumina to form a tenacious bond therebetween.
In general, the time required to form this eutectic melt will be that time
necessary to form a good bond without oxidizing the copper. A more
detailed relationship between copper thickness and time at an elevated
temperature of between 1065.degree. and 1075.degree. C. is presented in
Table II of the above noted U.S. Pat. No. 3,993,411.
Table II noted above illustrates the relationship between copper thickness,
non-metallic refractory material thickness and firing time in the furnace,
i.e., the time at which the metal-nonmetal materials remain in the
furnace. From this table, it is readily apparent that the firing time
increases with the metal thickness, although there does not appear to be a
linear relationship between the two.
Though the invention has been described with respect to specific preferred
embodiments thereof, many variations and modifications will immediately
become apparent to those skilled in the art. It is therefore the intention
that the appended claims be interpreted as broadly as possible in view of
the prior art to include all such variations and modifications.
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